The positive and negative electrodes of lithium-ion secondary batteries are provided with a material (active material) that is capable of reversibly storing and releasing lithium ions (Li ions). Charging and discharging are elicited through traffic of lithium ions between the positive and negative electrodes. Such lithium-ion secondary batteries afford, in particular, high energy densities at a low weight. For this reason, these batteries hold potential as secondary batteries that are suitable for high-output power sources installed in vehicles. Lithium ion secondary batteries have been gaining importance, in that they can be used not only as power sources installed in vehicles, but also as small power sources, for instance in computers and mobile terminals, and as large power sources such as storage batteries for residential use.
Representative examples of active materials that are used in the electrodes (typically, the positive electrode) of lithium-ion secondary batteries include, for instance, complex oxides that comprise lithium and a transition metal element. As the abovementioned transition metal element, there is preferably used, for instance, a lithium complex oxide (nickel-containing lithium complex oxide) comprising at least nickel (Ni) and having a layered structure. Patent Literature 1 to 5 disclose technologies relating to active materials of lithium-ion secondary batteries.
Patent Literature 1 discloses the feature of using, as a positive electrode active material, a powder of a lithium-containing complex oxide having a DBP absorption number ranging from 20 mL to 40 mL per 100 g.
Patent Literature 2 discloses a lithium-ion secondary battery having a positive electrode mixture layer that comprises a positive electrode active material and a conducting agent. The conducting agent in the lithium-ion secondary battery disclosed in Patent Literature 2 takes up 7 mass % to 13 mass % of the positive electrode mixture layer. The porosity of the positive electrode mixture layer ranges from 35% to 55%. The concentration of a lithium salt in a nonaqueous electrolyte solution ranges from 102 mol/L to 2 mol/L. Patent Literature 2 indicates that, as a result, it becomes possible to suppress voltage drops even during discharge at high load. Further, Patent Literature 2 indicates that it is possible to provide a lithium-ion secondary battery having excellent output characteristic.
Patent Literature 3 discloses a method for producing a paste for a nonaqueous electrolyte secondary battery positive electrode plate. In the disclosed method for producing a paste for positive electrode plates of nonaqueous electrolyte secondary batteries, the mass ratio of conducting agent for the positive electrode ranges from 1.0 part by weight to 5.0 parts by weight with respect to 100 parts by weight of positive electrode active material, during production of the paste. Patent Literature 3 discloses a feature wherein a mixed powder resulting from combining the positive electrode active material and a conducting agent at a predetermined material input ratio, during production of the paste, exhibits a DBP (dibutyl phthalate) absorption number ranging from 15 mL/100 g to 40 mL/100 g (herein, “mL/100 g” denotes the absorption amount of DBP per 100 g of powder) as measured in accordance with method A prescribed in JIS K6217-4 “Test method of carbon black for rubber, fundamental properties”.
Patent Literature 4 discloses a lithium-ion secondary battery wherein the porosity of a positive electrode mixture ranges from 21% to 31%.
Patent Literature 5 discloses a feature wherein the density of a positive electrode mixture layer is 3.5 g/cm3 or higher and the porosity 25% or lower.